An uninterruptible power supply (UPS) may be used to provide emergency power in situations where the main power supply fails or performs in an unusual manner. The UPS is designed to switchover in a near instantaneous or instantaneous fashion, due to the nature of the batteries used and the circuitry of the UPS. A UPS can be used to deal with a number of unusual events occurring at the mains power supply, such as: power failure; surge; sag; spikes in the power supply; noise; frequency instability; harmonic distortion; etc.
A UPS can be used to protect any type of equipment, however generally a UPS is most often found in computers, data centres, telecommunication systems and any other electrical equipment which could cause serious consequences such as damage to the person or to a business interest if there were no power.
A UPS can take many different forms and relate to various different technologies. The most common general categories of UPS are online, line interactive, and standby. Each of these is well-known in the art as are the other alternatives such as hybrid topologies and ferro-resistant technologies.
A UPS includes three main power converters. A DC to AC converter is used to supply the load with a clean AC voltage. That DC to AC converter is called an inverter and could be supplied by two different input sources. Firstly, the inverter could be supplied by an AC to DC converter used to convert an AC input source voltage into a DC voltage, which in turn is used to supply an output DC to AC converter. Secondly, a bi-directional DC to DC converter could be used in any circumstance where there is a failure in the input AC source to thus supply the DC to AC converter from the batteries.
The three power converters in a UPS are controlled by a control board in order to reach objectives defined for the UPS. One such objective may be summarised as: supplying an output customer load with a high quality voltage, whatever the state of the input AC voltage source, in a secure and safe manner. To achieve these objectives, certain physical signals need to be controlled using regulation algorithms or the like. On each converter, output voltage and driven current are regulated. Generally, all regulation algorithms used to control output voltage and driven currents in a UPS are embedded in the same control board.
One problem with using the same control board is that the control board has to be re-designed for any new UPS requirements (power topology, product architecture, etc.). This problem occurs each time there is a new project. The re-use of control boards from older products is difficult due to a high level of reliability expected for the control and UPS specific requirements.
In order to design a new product in which controls can be re-usable in future products, regulations or control functions have been classified into two categories. The first category contains regulations common to all UPS product requirements; the second category contains regulations specific to each new UPS product requirement.
In order to improve future re-use of control, the two categories of regulation or control function have to be embedded in two different circuit boards. The first is a common re-usable circuit board containing first category regulation (common to all UPSs) and the second is a specific circuit board containing second category regulation (specific to one particular UPS), which will be re-designed for each new product.
Generally, the specific circuit boards are of an on-chip design since the number of manufactured units will be low and may only be relevant to one product.
The common re-usable circuit board will contain all the common regulations and complex algorithms. As the manufactured number will be high, the cost optimisation will be a lower priority than is the case for the specific board.
The present invention provides a way of implementing current regulation in a cost effective manner in a specific control circuit board.
A number of proposals have been tried in the past in order to address the problem. One proposal relates to a fixed frequency linear analogue current controller, however this has been found to be difficult to tune, with a limited reliability and flexibility. Another proposal relates to a fixed frequency linear digital current controller. This requires expensive and fast A to D converters which have limited dynamic range. As a result, the solution is expensive and has reduced performance. A third proposal relates to a variable frequency sliding mode controller, with or without a variable hysteresis band. This is not suitable for situations where fixed switching frequencies are demanded. A fourth proposal relates to a fixed frequency peak current controller. However, this gives rise to dynamic error and requires a complex analogue implementation to deal with all the compensation requirements.
Generally, current controllers can be classified into two main types, these are linear and non-linear.
Linear current controllers compare a current reference with a current measure and compute an error. A linear function, usually comprising gain or integration, is applied to the error and the result of the linear function is provided to a control board to control the current. A binary command signal is generated by the control board to control the output current of an inverter and thereby controls a supply current. The command signal has a pulse width that is altered to control the inverter, but is set to 0 at a regular period and is therefore considered a “fixed frequency”.
Non-linear controllers determine the current error in the same way and switch a command control signal between 1 and 0 if the error exceeds certain boundary conditions. If the error is greater than an upper threshold then the command is set to reduce the current, and if the error is less than a lower threshold then the command is set to increase the current. In this case the command signal is considered to be of “variable frequency” as it is switched based only on the threshold values and without any timing considerations.
As previously mentioned, hybrid controllers also exist, which use non-linear control with a fixed frequency command. In this case there are two constraints on the command: staying within the error thresholds and the period of the command. An algorithm used in the control board considers the two constraints and decides whether to switch the command signal.
The above mentioned controllers are useful in many situations, but they do not address all the problems of the prior art.